Insulating containers are employed in the handling, storage and use of liquified gases such as liquid nitrogen, oxygen, hydrogen and helium. The very low temperatures, for example -196.degree. C. (77 K) for nitrogen, require effective insulating barriers to prevent heat incursion and consequential vaporization and loss of the liquified gas. Another common problem is that the outer surface of the containers becomes cold enough to cause condensation or even frosting of the surface. The wet or frosted surface not only is a nuisance but can lead to more serious problems such as corrosion of the surface or damage to articles contacting the moisture.
A variety of insulation techniques and devices currently exist. One such device is a vacuum container in which the space between inner and outer walls is evacuated. This must be cylindrical and is relatively expensive, especially for larger containers. Glass vacuum systems are fragile, and stainless steel systems are even more costly. A variation of the vacuum approach is called multi-layered insulation where a layering of two or more polymer films separated by close placements of button-shaped members with a vacuum between the polymer films. This is suitable where flexibility is desired, for example for shaped surfaces, but not for a rigid, functional surface, and also is quite expensive.
Insulation is also commonly provided by packed fibers of glass or mineral, and blocks of closed-pore polymer such as polystyrene foam (e.g. Styrofoam.TM.). The fibers or foams are used to prevent air circulation in the insulating space, since such circulation causes the air to convey heat between the walls bounding the space. Such insulation is economical and is used on containers for liquified gas, but it often is not sufficiently effective. For example, such containers using liquid nitrogen typically have condensation or frosting on the outside. Heating tape has been used for the outer surface to reduce the condensation, but this often is insufficient.
Another form of insulation, for more moderated temperatures, uses windows of the type commonly known as Thermopane.TM. with two or three panes of glass to enclose one or two spaces filled with a dry gas. These are designed for windows of buildings for visibility and passage of light, as well as to insulate the building interior from external hot or cold weather temperatures.
Certain calorimeters sometimes are adapted to utilize liquid nitrogen. These instruments are used for obtaining measurements on thermal characteristics of samples. One such calorimeter is a differential scanning calorimeter (DSC) for making measurements on a pair of samples in such a manner that a test sample is compared to another test sample or a reference sample, such as disclosed in U.S. Pat. No. 3,263,484. The samples are supported in an insulated container. A power supply provides power to each of the samples and temperatures are measured. In a DSC, differential power and temperature between samples are determined for comparison of heating characteristics such as phase changes. A furnace or other heating vessel is placed over the sample region for heating to provide a range of temperatures for the measurements. Liquid nitrogen, located in a reservoir below the sample support, may be used to extend DSC measurements to a lower temperature range. Such a system has used thick styrene foam blocks for insulation against the cold, but surface condensation has still been a significant problem, even with heating tape.
An object of the invention is to provide an improved system of thermal insulation for liquified gas such as liquid nitrogen. A further object is to provide such a system with reduced susceptibility to condensation of moisture on outer surfaces. A preferred object is to provide such a system incorporated into a calorimeter.